We have been using Java 8 in production for over a year now and would like to share some of our experiences and best practices. Java 8 helps us to adopt a more functional style in our programs, and this talk will discuss how to write code that is both elegant, readable and efficient. During the talk I go in depth on some of the new language features in Java 8, and also touch on some nice additional utilities. Some of the topics include: - Lambdas vs. Method Handles - Streams + Optional = true? - parallelStream FTW? - To mutate or not to mutate

1. Java 8 and .
1
javaBin Oslo – 12.05.2015
Fredrik Vraalsen
fredriv vraalsen@iterate.no

2. 2

3. Java 8 – what’s new?
Lambdas
Method handles
Extension methods
Streams
Optional
3

4. Java 8 – what’s new?
Lambdas (anonymous functions)
Method handles
Extension methods
Streams
Optional
4

5. 5

6. Java 7 vs 8
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
});
6

7. Java 7 vs 8
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
});
vs
sort(people, (x, y) -> x.getName().compareTo(y.getName()));
7

8. Java 7 vs 8
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
});
vs
sort(people, comparing(person -> person.getName()));
8

9. Java 7 vs 8
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
});
vs
sort(people, comparing(Person::getName));
9

10. Java 7 vs 8
Collections.sort(people, new Comparator<Person>() {
public int compare(Person x, Person y) {
return x.getName().compareTo(y.getName());
}
});
vs
people.sort(comparing(Person::getName));
10

11. Streams
© Fredrik Vraalsen 2008

12. Old fashioned imperative
12
List<RoadData> filtered = new ArrayList<>();
int count = 0;
for (Iterator<RoadData> i = roadData.iterator();
i.hasNext() && count < 10; ) {
RoadData data = i.next();
if (nameQuery.matches(data.getName())) {
filtered.add(data);
count++;
}
}

13. Streams
13
roadData.stream()
.filter(data -> nameQuery.matches(data.getName()))
.limit(10)
.collect(toList());

14. Streams – pipelines
14
roadData.stream()
.filter(data -> nameQuery.matches(data.getName()))
.limit(10)
.collect(toList());
cat roadData.txt | grep … | head > output.txt

15. Learn you a Stream API
15

16. Map<String, List<Article>> articlesByCategory =
Learn you a Stream API
16

17. Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
Learn you a Stream API
17

18. Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
.map(transport -> convertToArticle(transport))
Learn you a Stream API
18

19. Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
.map(transport -> convertToArticle(transport))
.collect(groupingBy(article -> article.getCategory()));
Learn you a Stream API
19

20. Method handles
20
Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
.map(transport -> convertToArticle(transport))
.collect(groupingBy(article -> article.getCategory()));

21. Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
.map(this::convertToArticle)
.collect(groupingBy(article -> article.getCategory()));
Method handles
21

22. Method handles
22
Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
.map(this::convertToArticle)
.collect(groupingBy(Article::getCategory));

23. Naming, caching, counting
23
Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
.map(this::convertToArticle)
.collect(groupingBy(Article::getCategory));

24. Naming, caching, counting
24
Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
.map(this::transportToArticle)
.collect(groupingBy(Article::category));

25. Extract functions
25
Map<String, List<Article>> articlesByCategory =
articleTransports.stream()
.map(transportToArticle)
.collect(groupingBy(Article::category));
Function<ArticleTransport, Article> transportToArticle =
transport -> convertToArticle(transport);

26. Lambdas everywhere!!!1!
links.stream()
.map(link -> link.build())
.forEach(abderaElement::addLink);
26

27. Old school ifs and fors
links.stream()
.map(link -> link.build())
.forEach(abderaElement::addLink);
for (LinkBuilder lb : links) {
abderaElement.addLink(lb.build());
}
27

28. Parsing nested JSON
"suggest": {
"tag_suggest": [
{
"length": 5,
"offset": 0,
"options": [
{
"freq": 25,
"score": 0.8,
"text": "fakta"
}
],
"text": "fanta"
}
]
} 28

29. Lambdas everywhere!!!1!
List<String> terms = new ArrayList<>();
suggestions.getAsJsonObject().getAsJsonArray(“tag_suggest”)
.forEach(tag -> tag.getAsJsonObject().getAsJsonArray("options")
.forEach(option -> terms
.add(option.getAsJsonObject().getAsJsonPrimitive("text")
.getAsString())));
29

30. Old school ifs and fors
List<String> terms = new ArrayList<>();
for (JsonElement tag : suggestions.getAsJsonObject().getAsJsonArray("tag_suggest")) {
for (JsonElement option : tag.getAsJsonObject().getAsJsonArray("options")) {
terms.add(option.getAsJsonObject().getAsJsonPrimitive("text").getAsString());
}
}
30

31. Old school ifs and fors
List<String> terms = new ArrayList<>();
for (JsonElement tag : getAll("tag_suggest", suggestions)) {
for (JsonElement option : getAll("options", tag)) {
terms.add(getString("text", option));
}
}
31

32. List<String> terms = getAll("tag_suggest", suggestions).stream()
.flatMap(tag -> getAll("options", tag).stream())
.map(option -> getString("text", option))
.collect(Collectors.toList());
32
Lambdas everywhere!!!1!

33. Readability ≫ Style / LOC
Always code as if the person who ends up maintaining your code
is a violent psychopath who knows where you live.
http://c2.com/cgi/wiki?CodeForTheMaintainer

34. OH NOES
NullPointerException!!?!
© Fredrik Vraalsen 201

35. Optional<T>
Wrapper object
Make explicit if a value is present or absent (empty)
35

36. Optional example
public Optional<Image> getImage(Article article)
---
36

37. Optional example
public Optional<Image> getImage(Article article)
---
Optional<Image> image = getImage(article);
37

38. Check presence
public Optional<Image> getImage(Article article)
---
Optional<Image> image = getImage(article);
if (image.isPresent()) {
doSomething(image.get());
}
38

39. Check presence
public Optional<Image> getImage(Article article)
---
Optional<Image> image = getImage(article);
image.ifPresent(img -> doSomething(img));
39

40. Check presence
public Optional<Image> getImage(Article article)
---
getImage(article).ifPresent(image -> doSomething(image));
40

41. Default / fallback value
public Optional<Image> getImage(Article article)
---
Image image = getImage(article).orElse(defaultImage);
41

42. Optional in / Optional out
public Optional<String> getCaption(Optional<Image> image)
---
Optional<Image> image = getImage(article);
Optional<String> caption = getCaption(image);
42

43. Optional in / Optional out
public String getCaption(Image image)
---
Optional<Image> image = getImage(article);
Optional<String> caption = ???
43

44. Reaching into the void …
public String getCaption(Image image)
---
Optional<Image> image = getImage(article);
Optional<String> caption = image.map(img -> getCaption(img));
44

45. Reaching into the void …
public String getCaption(Image image)
---
Optional<String> caption = getImage(article)
.map(img -> getCaption(image));
45

46. Bye bye NullPointerException?
Marker – this may not return/contain value
Public APIs
Not a general solution to NPEs
46

47. Optional + Streams = true?
47

48. Optional + Streams = true?
48
myStream.map(v -> functionReturningOptional(v))

49. Optional + Streams = true?
49
myStream.map(v -> functionReturningOptional(v))
Stream<Optional<T>>

50. Optional + Streams = true?
50
myStream.map(v -> functionReturningOptional(v))
Stream<Optional<T>> Stream<T> ???

51. Guava Optional
51
Iterable<T> presentInstances(Iterable<Optional<T>> optionals)

52. Optional + Streams = true?
52
public Optional<Image> getImage(Article article)
---
List<Image> images = articles.stream() // Stream<Article>

53. Optional + Streams = true?
53
public Optional<Image> getImage(Article article)
---
List<Image> images = articles.stream() // Stream<Article>
.map(article -> getImage(article)) // Stream<Optional<Image>>

54. Optional + Streams = true?
54
public Optional<Image> getImage(Article article)
---
List<Image> images = articles.stream() // Stream<Article>
.map(article -> getImage(article)) // Stream<Optional<Image>>
.filter(Optional::ifPresent)
.map(Optional::get) // Stream<Image>

55. Optional + Streams = true?
55
public Optional<Image> getImage(Article article)
---
List<Image> images = articles.stream() // Stream<Article>
.map(article -> getImage(article)) // Stream<Optional<Image>>
.filter(Optional::ifPresent)
.map(Optional::get) // Stream<Image>
.collect(toList());

56. flatMap to the rescue!
flatMap = map + flatten
Stream<Stream<T>> Stream<T>
56

57. flatMap to the rescue!
flatMap = map + flatten
Stream<Optional<T>> Stream<T> ???
57

58. flatMap to the rescue!
Easy to create helper functions
Stream<T> toStream(Optional<T> optValue)
58
http://stackoverflow.com/questions/22725537/using-java-8s-optional-with-streamflatmap

59. Optional + Streams = true?
59
public Optional<Image> getImage(Article article)
---
List<Image> images = articles.stream() // Stream<Article>
.map(article -> getImage(article)) // Stream<Optional<Image>>
.filter(Optional::ifPresent)
.map(Optional::get) // Stream<Image>
.collect(toList());

60. Optional + Streams = true
60
public Optional<Image> getImage(Article article)
---
List<Image> images = articles.stream() // Stream<Article>
.flatMap(article -> toStream(getImage(article))) // Stream<Image>
.collect(toList());

61. Optional + Streams = true
61
public Optional<Image> getImage(Article article)
---
List<Image> images = articles.stream() // Stream<Article>
.flatMap(article -> getImage(article).stream()) // Stream<Image>
.collect(toList());
Java 9
https://bugs.openjdk.java.net/browse/JDK-8050820

62. Take one stream and pass it around…
62
public Stream<Image> getImage(Article article)
---
List<Image> images = articles.stream() // Stream<Article>
.flatMap(article -> getImage(article)) // Stream<Image>
.collect(toList());

63. No method handle for you!
63
private Article elementToArticle(Map contentById, Element el)
---
List<Article> articles = elements.stream()
.map(el -> elementToArticle(contentById, el))
.collect(toList())

64. 64
http://en.wikipedia.org/wiki/Malabar_matthi_curry#/media/File:Meen_curry_2_(cropped).JPG

65. Yummy currying!!
65
Mapper mapper = new Mapper(contentById);
---
List<Article> articles = elements.stream()
.map(el -> mapper.elementToArticle(el))
.collect(toList())

66. Yummy currying!!
66
Mapper mapper = new Mapper(contentById);
---
List<Article> articles = elements.stream()
.map(mapper::elementToArticle)
.collect(toList())

67. Performance
© Fredrik Vraalsen 2012

68. Streams – performance
List<Article> frontpageArticles =
frontpage.getArticleIds().stream()
.map(id -> fetchArticle(id))
.collect(toList());
68

69. This one goes to 11!
List<Article> frontpageArticles =
frontpage.getArticleIds().parallelStream()
.map(id -> fetchArticle(id))
.collect(toList());
69

70. Starvation
Common F/J thread pool
Workarounds?
Execute within explicit F/J pool
Use CompletableFuture
70

71. CompletableFuture
Chaining of async futures and actions
Waiting for all or any future(s)
Explicitly complete (like Promise)
Control of executor service
71
http://blog.krecan.net/2014/03/18/how-to-specify-thread-pool-for-java-8-parallel-streams/
http://www.nurkiewicz.com/2013/05/java-8-completablefuture-in-action.html

72. To parallelStream or not to …
Batch?
parallelStream FTW!
Interactive? Concurrency?
CompletableFuture FTW!
72

73. Taking the happy path!
Try<RestResponse<ResolvedUri>> resolveURI(URI uri)
---
public ResolvedSectionUri resolveSectionUri(String uri) {
return client.resolveURI(uri))
??? Try<RestResponse<ResolvedUri>>
}
73

74. Taking the happy path!
Try<RestResponse<ResolvedUri>> resolveURI(URI uri)
---
public ResolvedSectionUri resolveSectionUri(String uri) {
return client.resolveURI(uri))
.map(response -> response.getEntity())
??? Try<Optional<ResolvedUri>>
}
74

75. Taking the happy path!
Try<RestResponse<ResolvedUri>> resolveURI(URI uri)
---
public ResolvedSectionUri resolveSectionUri(String uri) {
return client.resolveURI(uri))
.map(response -> response.getEntity().get())
??? Try<ResolvedUri>
}
75

76. Taking the happy path!
Try<RestResponse<ResolvedUri>> resolveURI(URI uri)
---
public ResolvedSectionUri resolveSectionUri(String uri) {
return client.resolveURI(uri))
.map(response -> (ResolvedSectionUri) response.getEntity().get())
??? Try<ResolvedSectionUri>
}
76

77. Taking the happy path!
Try<RestResponse<ResolvedUri>> resolveURI(URI uri)
---
public ResolvedSectionUri resolveSectionUri(String uri) {
return client.resolveURI(uri))
.map(response -> (ResolvedSectionUri) response.getEntity().get())
.orElse(null);
}
77

78. Taking the happy path!
Try<RestResponse<ResolvedUri>> resolveURI(URI uri)
---
public ResolvedSectionUri resolveSectionUri(String uri) {
return client.resolveURI(uri))
.map(response -> (ResolvedSectionUri) response.getEntity().get())
.orElseGet(() -> {
log.warn("Cannot resolve section from URI: {}", uri);
return null;
});
}
78
https://github.com/lambdista/try

79. More cool stuff in Java 8
String join
Collection removeIf (≈ filter)
Map getOrDefault, putIfAbsent, replace, forEach, etc.
java.util.Optional
java.util.stream.Collectors
count, sum, average, min, max, groupingBy, etc.
java.nio.file.Files lines
79

80. Functional programming
Simpler code
More robust
Better performance

81. Higher level
Declarative
Less verbose
Less bugs?
81
List<RoadData> filtered = new ArrayList<>();
int count = 0;
for (Iterator<RoadData> i = roadData.iterator();
i.hasNext() && count < 10; ) {
RoadData data = i.next();
if (nameQuery.matches(data.getName())) {
filtered.add(data);
count++;
}
}
roadData.stream()
.filter(data -> nameQuery.matches(data.getName()))
.limit(10)
.collect(toList());
vs.

82. Pure functions
82
Data In Data Out
Transformation
No external interactions

83. Functional core
83
————- —— —
—— —- — —- —-
—————
———
—— —- — —- —
————
—— — — — ——-
——- —- — ————
—- ——— ———
———- —— ——
—— - —-
———- — —- — -
Pure functionsI/O I/O

84. Building blocks
© Fredrik Vraalsen 201

85. What’s missing?
© Fredrik Vraalsen 2012

86. What’s missing?
Immutability
Value types
Data structures (lists, maps, etc.)
Concurrency
86

87. What’s the big deal?
Functional programming is all about values!
And transformations (functions) computing new values
Parallellism vs. Concurrency
Robustness
Testability
87

88. Some help to be found
Immutable collections
Google Guava, FunctionalJava, clj-ds
Concurrency mechanisms
Akka (Actors, STM)
88

89. github.com/krukow/clj-ds
PersistentVector<Person> people = Persistents.vector(
new Person("Fredrik", 39),
new Person("Hedda", 3));
89

90. github.com/krukow/clj-ds
PersistentVector<Person> people = Persistents.vector(
new Person("Fredrik", 39),
new Person("Hedda", 3));
PersistentVector<Person> morePeople =
people.plus(new Person("Johannes", 5));
90

91. github.com/krukow/clj-ds
PersistentVector<Person> people = Persistents.vector(
new Person("Fredrik", 39),
new Person("Hedda", 3));
PersistentVector<Person> morePeople =
people.plus(new Person("Johannes", 5));
morePeople.stream()
.forEach(p -> System.out.println(p.getName()));
91

92. Why use X instead?
Java 8 ready for enterprise dev?
JBoss AS, WebSphere – Nope
WildFly, GlassFish, WebLogic, Jetty, Tomcat – OK?
Important things are still missing from Java 8
Clojure and Scala available on JDK 6+!
92

93. Questions?
© Fredrik Vraalsen 2012
fredriv
vraalsen@iterate.no